1. Field
The present disclosure relates to a phase shift transformer, and more particularly to a phase shift transformer for use in a multi-level medium voltage inverter.
2. Background
In general, a multi-level medium voltage inverter is an inverter having a root mean square value of an inputted line-to-line voltage ranging from 600 v or over, and an output phase voltage thereof is multi-leveled. The multi-level medium voltage inverter is configured to drive a motor having capacity of several kW to several MW in the fields such as a fan, a pump, a compressor, a traction device, a hoist and a conveyor.
A conventional medium voltage inverter may require a phase shift transformer at an input terminal for providing galvanic isolation, reducing harmonics, and supplying an input voltage to each power cell unit. The phase shift transformer, as one of constituent parts of the medium voltage inverter, takes a lion's share of volume and weight in the medium voltage inverter, such that its design is very important.
However, the conventional phase shift transformer, generally having an integrated configuration, suffers from drawbacks such as non-existence in degree of freedom in terms of design due to the integrated structure. Now, the conventional phase shift transformer will be described.
FIG. 1 is a schematic structural view of a multi-level medium voltage inverter according to prior art.
Referring to FIG. 1, a conventional multi-level medium voltage inverter receives an input power from an input power unit (120) outputting a voltage whose root mean square value of an inputted line-to-line voltage is 600 volt or over, and converts the voltage to a load of a three-phase motor (130).
A primary winding of a conventional phase shift transformer (111) is composed of a three-phase Y connection, and a secondary winding is composed of a total of 12 windings, three windings each having a phase difference of −15°, 0°, 15°, 30° relative to the primary winding. Structure of the secondary winding is determined by the number of unit power cells (112). It can be noted from FIG. 1 that the inverter (110) has two unit power cells for each phase of the motor (130). That is, outputs of A1 and A2 power cells (112a, 112b) are connected in series to output an ‘a’ phase voltage of the three phase motor (130), B1 and B2 power cells (112c, 112d) output a ‘b’ phase voltage, and C1 and C2 power cells (112e, 112f) output a ‘c’ phase voltage.
The A1, B1 and C1 power cells (112a, 112c, 112e) are connected to an output having 15° and 0° phases, and A2, B2 and C2 power cells (112b, 112d, 120 are connected to an output having 15° and 30° phases.
FIG. 2 is a configuration view of a power cell in FIG. 1. Referring to FIG. 2, a rectifying unit (210) receives two three-phase powers from the phase shift transformer (111) and rectifies the power to a DC voltage. To this end, the rectifying unit (210) includes two diode rectifiers. An output of the rectifying unit (210) is connected to serially-connected DC link capacitors, and each of the two DC link capacitors of a DC unit (220) has a same capacitance. An inverter unit (230) synthesizes output voltages of the DC unit (220) and an outputted line-to-line voltage is 5 levels.
Meantime, the convention medium voltage inverter of FIG. 1 may be configured as in FIG. 3. FIG. 3 is another configuration view of FIG. 1, and has the same configuration as in FIG. 1 except for configuration of a power cell (113).
The power cell (113) of FIG. 3 can synthesize output voltages of 5 levels. A D1 power cell (113a) can output an ‘a’ phase voltage of the motor (130), an E1 power cell (113b) can output a ‘b’ phase voltage of the motor (130), and an F1 power cell (113c) can output a ‘c’ phase voltage of the motor (130).
FIG. 4 is a configuration view of a power cell of FIG. 3.
Referring to FIG. 4, a rectifying unit (410) includes four diode rectifiers, and operation of an inverter unit (430) is same as in FIG. 2. However, the unit power cells in FIGS. 2 and 4 are different in rated voltage and rated current of power device used in response to required output. An output of the phase shift transformer (111) is inputted to the rectifying unit (410), and an output of the rectifying unit (410) is reflected on a DC unit (420).
First of all, an operation of the inverter unit (230) of FIG. 1 will be described.
Each leg of the inverter unit (230) in FIG. 2 includes four serially-arranged switches (230a, 230b, 230c, 230d), and an output voltage is defined by operation of each switch.
Switching operations of switches of 230a and 230b are complementary and switching operations of switches of 230c and 230d are also complementary. Thus, if each voltage of serially-connected DC link capacitors of the DC unit (220) is defined as E, and if switches of 230a and 230b are turned on, switches of 230c and 230d are turned off, and outputted pole voltage is E. Furthermore, if switches of 230a and 230c are turned on, switches of 230b and 230d are turned off, and pole voltage at this time is zero. Likewise, if switches of 230a and 230b are turned off, switches of 230c and 230d are turned on, and pole voltage at this time is −E.
According to the pole voltages thus defined, line-to-line voltages outputted by each of the unit power cells (112) of FIG. 1 are respectively 5 levels of 2E, E, 0, −E and −2E. Due to the line-to-line voltages outputted by each power cell (112) being defined as five levels, voltages synthesizable by A1 and A2 power cells (112a, 112b) of FIG. 1 are nine levels of 4E, 3E, 2E, E, 0, −E, −2E, −3E and −4E, and line-to-line voltages outputted to the motor (130) are 17 levels of 8E, 7E, 6E, 5E, 4E, 3E, 2E, E, 0, −E, −2E, −3E, −4E, −5E, −6E, −7E, and −8E.
Now, operation of the phase shift transformer (111) of FIGS. 1 and 3 will be described.
The phase shift transformer (111) applies an electrically-insulated 3-phase power to each power cell (112, 113) from an inputted 3-phase power. A primary winding of the phase shift transformer (111) is a Y-connection or a delta (Δ)-connection, and a secondary winding outputs a power with a shifted phase and magnitude adequate to requirement of the unit power cells (112, 113).
At this time, a number of an output from the secondary winding of the phase shift transformer (111) is same as a number of the diode rectifiers of rectifying units (210, 410) at the unit power cells (112, 113), and can be defined by the following Equation 1.Nsec=3NunitNdiode  [Equation 1]where, Nsec is the number of the output from the secondary winding of the phase shift transformer (111), Nunit is the number of unit power cells (112, 113) connected to each phase of the motor (130), and Ndiode is the number of diode rectifiers included in one unit power cell (112, 113).
For example, because Nunit is 2 in configurations of FIGS. 1 and 2, and Ndiode is 2, Nsec becomes 12, and because Nunit is 1, Ndiode is 4 in configurations of FIGS. 3 and 4, Nsec becomes 12.
A phase shift angle of the secondary winding of the phase shift transformer (111) may be obtained by the following Equation 2.
                              α          sec                =                              360                          2              ⁢                                                          ⁢                              N                sec                                              ⁡                      [            degree            ]                                              [                  Equation          ⁢                                          ⁢          2                ]            where, αsec is the phase shift angle between secondary windings. For example, if Nsec becomes 12 as in FIGS. 1 and 3, the phase shift angle between secondary windings is 15°. An output voltage of each secondary winding from the phase shift angle between secondary windings thus determined is such that a phase relative to an input voltage at the primary winding is changed as much as the phase shift angle.
However, the abovementioned conventional multi-level medium voltage inverter is configured with a phase shift transformer in a single unit. The phase shift transformer structured with a single unit suffers from a disadvantage in that, because a required output must be satisfied by one transformer, size and weight of the transformer increase. Another disadvantage is that no layout freedom is available to increase an entire system volume-wise and to increase weight as well. Still another disadvantage is that if a problem occurs in the primary winding in the conventional phase shift transformer of single unit, an entire system is rendered inoperable.